This section provides background information related to the present disclosure which is not necessarily prior art.
A spark ignition engine includes a spark plug that protrudes into a combustion chamber of the engine and provides an insulated electrode with a fixed spark gap to ground. An ignition coil provides the energy required for electrical current to arc across the spark gap igniting an air-fuel mixture within the combustion chamber and causing combustion.
During the combustion phase of the engine, heat and pressure cause ionization of gases within the combustion chamber. The degree of ionization is measured by applying a voltage across an electrical gap inside the combustion chamber and measuring the resulting current, which is referred to as an ionization current. The ionization current indicates the quality of the combustion, including the occurrence of knock and misfire. An engine control module analyzes a signal indicative of the ionization current and operates as part of a closed-loop combustion control to optimize the combustion within the chamber.
To detect the ionization current, an ionization detector utilizes the existing spark plug as the ionization sensing electrical gap inside the combustion chamber. However, the voltage at the spark plug terminal is extremely high during the spark event (e.g., 30 kV). Therefore, the ionization detector cannot be directly connected at the spark plug terminal. Instead, the ionization detector is connected to the spark plug by way of a secondary winding of the ignition coil.
The inductance characteristic of the second winding creates a parasitic low-pass filter, and effectively filters certain frequencies. A low energy ignition coil has a low enough secondary inductance to pass knock frequencies. However, the trend in automotive applications is toward higher energy coils that require higher inductance resulting in attenuation of the ionization signal, thereby preventing the engine control module from detecting the quality of the combustion, such as the occurrence of knock.